Question

BiCl3 is less covalent than PCl3.why?

Answer 1

According to the Fajan rule, the smaller the cation, the strong the covalent bond it form. So P is smaller in size as compare to Bi and thus form more covalent structure. 

Answer 2

P3 + is smaller in size as compared to Bi3 +  

If we look at the Fajan's rule, the smaller the cation the higher will be the covalent nature of the bond. 

So BiCl3 is less covalent than PCl3